added DiffTask Entity and Mapper

lib/Db/DiffTask.php

@@ -0,0 +1,28 @@
+<?php
+namespace OCA\GroupfolderFilesystemSnapshots\Db;
+
+use JsonSerializable;
+
+use OCP\AppFramework\Db\Entity;
+
+class DiffTask extends Entity implements JsonSerializable {
+
+    protected $userId;
+    protected $groupfolderId;
+    protected $snapshotId;
+    protected $timestamp;
+
+    public function __construct() {
+        $this->addType('id','integer');
+        $this->addType('groupfolderId','integer');
+    }
+
+    public function jsonSerialize() {
+        return [
+            'id' => $this->id,
+            'groupfolderId' => $this->groupfolderId,
+            'snapshotId' => $this->snapshotId,
+            'timestamp' => $this->timestamp,
+        ];
+    }
+}